Generate \x{...} instead of \x... in regular expressions

[sungshik]

This PR fixes a small bug in the conversion of chars in regular expressions.

[DavyLandman]

Hmm, that is strange, the manual said the 1byte chars were allowed to be encoded as \xHH instead of \x{HH}

[sungshik]

Davy and I investigated this a bit further and:

• The docs state \xHH is for "encoded byte value", while \x{HH} is for "code point value"
• Only up to 7F, encoded bytes and code points coincide
• As a result, \xHH gives unexpected results beyond 7F (the bug of this PR) and \x{HH} should be used instead (the fix)